Such particle production can be viewed in
terms of the scalar field condensate \(\langle \chi ^2 \rangle \) . Light scalars are subject to quantum fluctuations of order \(H\)  [1]} so that a semi–classical field \(\chi \) 
experiences a random walk. As a result, a significant \(\langle \chi ^2 \rangle \)  can accumulate, for example, by the end of inflation and play the role of dark matter [2]},[3]}.
Again, this mechanism is purely gravitational. The consequent dark matter distribution is not correlated with the inflaton fluctuations, therefore
this possibility is subject to strict isocurvature constraints [3]}.
